GRADE 9 Integrated Science MIXTURES,ELEMENTS AND COMPOUNDS – Water hardness Notes

Integrated Science — Mixtures, Elements and Compounds

Subtopic: Water hardness (age 14, Kenya)

What is hard water?

Hard water is water that contains dissolved calcium (Ca2+) and magnesium (Mg2+) ions, often as their salts (bicarbonates, sulfates or chlorides). In Kenya many borehole and well waters are hard because they flow through rocks that contain calcium and magnesium.

Types of hardness

• Temporary hardness – caused mainly by dissolved bicarbonates such as calcium hydrogen carbonate, Ca(HCO3)2, or magnesium hydrogen carbonate. It can be removed by boiling.
• Permanent hardness – caused by sulfates, chlorides and nitrates of Ca2+ and Mg2+ (for example CaSO4). It is not removed by boiling.

Simple chemistry (clear steps)

When you boil water containing calcium hydrogen carbonate:

CaSO4 (calcium sulfate) does not decompose on boiling and so causes permanent hardness.

How hard water affects daily life (Kenyan examples)

• Tea and mandazi kettles build up scale (white crust) which reduces heating efficiency.
• Soaps and shampoos form scum and need more soap to get lather — more expense for households.
• Clothes may become stiff and pale; washing machines and water heaters get damaged by scale.
• Livestock drinking very hard water may be affected in taste or with deposit build-up in troughs.

Simple tests for hardness (classroom level)

1. Soap test (qualitative) — Add a fixed amount of liquid soap to a sample of water in a test tube or bottle and shake. Hard water produces little foam and shows scum; soft (distilled) water produces lots of foam easily.
2. Boiling test (distinguish temporary from permanent) — Boil the water. If a white precipitate or scale forms, part of the hardness is temporary (bicarbonates). If nothing forms, the hardness is likely permanent.

Ways to remove or reduce hardness (suitable for homes and schools)

• Boiling — removes temporary hardness by precipitating CaCO3. Useful for small amounts (kettle for tea).
• Adding washing soda (sodium carbonate, Na2CO3) — reacts with Ca2+ to give insoluble CaCO3, which can be filtered out:
  Ca2+ + CO3^2− → CaCO3(s) ↓
• Ion-exchange softeners (commercial) — replace Ca2+ and Mg2+ with Na+; used in homes or industries but require maintenance.
• Distillation and reverse osmosis — produce very soft water but are expensive for many households.

Simple classroom experiment: Test and soften hard water

Materials: two small clear bottles or test tubes, sample of local water, distilled water (control), liquid soap, kettle or flame, spoon, washing soda (sodium carbonate), filter paper or cloth, labels.

1. Label bottles A (sample water) and B (distilled water).
2. Add 20 ml of each water to the bottles.
3. Add 1 ml liquid soap to each, cap and shake for 20 seconds. Observe foam/soap scum and record which forms more lather.
4. Boil 50 ml of the sample water and observe any white precipitate (scale). Allow to cool and filter the precipitate.
5. To a fresh 20 ml of the sample water, add a small pinch of washing soda, stir and allow solids to settle then filter. Repeat the soap test on the filtered water to see if lather improves.
6. Record results and conclude whether hardness is temporary or permanent and whether washing soda helped.

Short revision questions

1. Define hard water.
2. Explain how boiling removes temporary hardness. Write the chemical equation.
3. Give two effects of hard water in homes.
4. Describe one cheap method to soften water and how it works.

Answers (brief)

• Hard water contains dissolved Ca2+ and Mg2+ ions.
• Boiling causes bicarbonates to decompose: Ca(HCO3)2 → CaCO3(s) + CO2(g) + H2O(l).
• Effects: scum when washing, scale in kettles, more soap needed.
• Washing soda (Na2CO3) reacts with Ca2+ to form insoluble CaCO3 which can be removed by filtering.